1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which perform printing by ejecting ink from a plurality of ejection openings to a printing medium. In detail, the present invention relates to an ink jet printing apparatus and an ink jet printing method which perform printing using a printing head equipped with a plurality of ejection opening arrays ejecting the same color ink.
2. Description of the Related Art
A printer or a copy machine and the like, or a printing apparatus used as an output device for composite electronics or a work station including a computer or a word processor is configured so that printing can be performed on a printing medium such as a paper or a plastic thin sheet based on printing information. The printing apparatus like this is classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, or the like. The printing apparatus of the ink jet type (ink jet printing apparatus) among printing apparatuses of such various printing types uses an ink jet printing head as a printing unit to perform printing by ejecting an ink toward the printing medium from a ejection opening provided in the printing head. The printing apparatus of such ink jet type has advantages that the printing head is easily downsized, that high resolution image can be formed rapidly, and that noise is small because of non-impact type.
The ink jet printing apparatus like this is roughly classified into two types of a serial type and a full line type depending on its printing method. The ink jet printing apparatus of the serial type uses a method to perform printing while scanning in a main scanning direction intersecting with a conveying direction of the printing medium (sub scanning direction). In this method, every time a printing movement in one time main scanning is finished, a movement in which the printing medium is conveyed by a predetermined amount is repeated, and thus the printing on all region of the printing medium is performed. On the other hand, the ink jet printing apparatus of the full line type uses a printing method to perform only a movement of the printing medium in the conveying direction upon printing. In the full line type, the printing on all region of the printing medium is performed by performing printing continuously for one line while conveying the printing medium by use of the printing head in which ejection openings are arranged across entire width of the printing medium. The ink jet printing apparatus of the latter full line type uses a printing method having a capability of printing with higher speed in comparison with the serial type. For example, the printing with a resolution of 600×600 dpi (dot/inch) for the printing of mono-color such as a sentence, or a high resolution printing with a resolution of 1200×1200 dpi or more for the printing of full-color picture like a photo can be also performed at a high speed of 60 pages or more per minute on the printing medium sized A4.
In the ink jet printing apparatus of the full line type, each ejection opening arranged across all width of printing region prints dots arranged along the conveying direction (hereinafter, the inverse direction of this direction is referred to as a main scanning direction). Accordingly, as with so called a multi-path printing which performs one line printing with a plurality of scanning in the serial type, one line is printed with a plurality of ejection openings, therefore, a variation of ejecting characteristic between the ejection openings cannot be reduced. Because of this, when the ejecting characteristic has a variation such that ejecting is not performed normally, and that an impact location displaces, this type has a defect that a fault in the picture such as stripe, stripe unevenness is easy of appearance. Originally, it is to be desired that all ejection openings shall be manufactured with no defect and an excellent accuracy, however, the number of the ejection openings is great, therefore, it is very hard to manufacture them with no defect and the excellent accuracy. For example, for performing the printing with the resolution of 1200 dpi in a sheet sized A3, it is necessary to provide about fourteen thousand units of the ejection openings (printing width 297 mm) in the printing head of the full line type, therefore, if they can be manufactured, manufacturing cost tends to increase because non-defective ratio is low. Because of this, in the printing head of the full line type, a constitution of so called connection heads so as to realize a long head by arranging relatively low cost short heads used for the printing of the serial type in such a manner that a plurality of units is connected in an arrangement direction of the ejection openings is general.
As one constitution reducing a problem of the above-mentioned variation caused by the printing head of the full line type, in order to weaken an influence applied to the printing with one ejection opening, a constitution in which dots in the main scanning direction shall be printed by not one ejection opening but a plurality of ejection openings is employed. This multi-array constitution of the ejection opening arrays can realize the printing with high-quality picture by reducing the variation of the ejecting characteristic between the ejection openings as well as a multi-path printing in the printing of the serial type. For example, a picture quality of the same level as 4-path printing in the printing of the serial type can be realized in such a way that the ejection opening array is constituted to be multiple as with a constitution in which 4-array ejection openings per one color are provided.
However, the present inventors examined and revealed that, when the printing is performed using the printing head of the multi-array constitution like this, uneven density varied with respect to the main scanning direction, so called conveyance unevenness tends to occur. Specifically, when the plural ejection opening arrays arranged in the main scanning direction are arranged mutually with a certain distance, it is found that the conveyance unevenness occurs remarkably as the distance between those ejection opening arrays becomes great. This is caused by a phenomenon in which the printing medium may be conveyed meanderingly, at that time, there exists a difference of ejection timing between the ejection opening arrays, and as a result the impact location displaces, resulting in the uneven density.
FIG. 13 is a drawing illustrating a situation performing the printing on a printing medium 5 conveyed in the arrow direction in the drawing with a printing head of 4-array constitution (array A, array B, array C, and array D) for the same ink color. Further, FIG. 14 is a graph showing a printing displacement (hereinafter, also referred to as X displacement) caused in such a manner that the printing medium is conveyed meanderingly in a state like a sine curve when the printing is performed with the printing head shown in FIG. 13.
As apparent from FIG. 13, each of four ejection opening arrays is arranged mutually in parallel with a fixed interval in the main scanning direction. In addition, an arrangement direction of ejection opening arrays is equivalent to the conveying direction of the printing medium (main scanning direction). Accordingly, when the printing is performed with ejection openings of four ejection opening arrays, printing timing is different for each array. Incidentally, actually, a dot of the same color is not printed overlapped so often at the same location of the printing medium. Normally, the dots are printed in order with four ejection openings so that they may be adjacent in the main scanning direction with a pitch depending on the resolution. However, since a mutual spacing between these four ejection opening arrays is far greater than the pitch of the above-mentioned adjacent dots, hereinafter, a location at which the dots are printed adjacently in the main scanning direction with these plural ejection openings is described as the same location for simplified description. When the printing is performed at the same location like this, if ejection timing is different for each ejection opening array, a printing displacement of each ejection opening array caused by the difference leads to a condition that phase is shifted as shown in FIG. 14.
A relation between a graph in FIG. 14 and a result of the printing will be described. In any graph of the arrays, there is occurred X-displacement within a range from +15 μm to −15 μm so as to draw a sine (sine wave) curve, and the phase is shifted by the amount corresponding to the difference in ejection timing. Regarding printing result, the printing result at the case in which a straight line is drawn without displacement in X is most preferable, and the uneven density does not occur either.
By the way, a portion in which a difference of X displacement among ejection opening arrays in each graph shown in FIG. 14 is small is each of inflection points of Q1, Q2, Q3, and Q4, and the printing results equivalent to portions near these inflection points Q1, Q2, Q3, and Q4 give almost favorable printing results. Further, in portions except the inflection points, namely, notwithstanding from plus to minus or from minus to plus, P1, P2, P3, and P4 which are large in X displacement variation amount, the printing becomes rough as a result that the impact location of the ink ejected is displaced. Accordingly, the printing result becomes a result with prominent uneven density in which dense portion and rough portion are generated alternately.
FIG. 15 shows that a difference of the X displacement between the array A and the array D and a difference of the X displacement between the array A and the array B in each main scanning position in FIG. 14 are represented in a graph. The comparison of FIG. 14 and FIG. 15 shows that the difference of the X displacement becomes small at a portion equivalent to the inflection points Q1, Q2, Q3, and Q4 in FIG. 14 in FIG. 15. The comparison also shows that the difference between the array A and the array B which are short in distance between the ejection opening arrays is smaller than the difference between the array A and the array D which are long in distance between the ejection opening arrays. Namely, the shorter the distance between the ejection opening arrays becomes, the less the uneven density becomes. Inversely, since the longer the distance between the ejection opening arrays becomes, the greater the X displacement becomes, the uneven density is generated remarkably accordingly. In particular, in a photographic output in which high image quality is required, the uneven density like this becomes unacceptable level.
As mentioned above, the shorter the distance between the ejection opening arrays becomes, the less the uneven density becomes. Namely, the uneven density generated in the printing result can be normally eliminated by performing the printing with one ejection opening array. However, in this case, an effect of so called multi-array constitution, in which when a certain ejection opening has a failure of miss ejecting, other ejection opening performs supplemental ejecting, can not be obtained, therefore, the printing result with high quality printing can not be obtained.
Incidentally, a meandering in the printing medium conveyance causing the above-mentioned problem, needless to say, needs not be a complete sine wave curve as mentioned above. Further, even when the meandering is generated in a part of the conveyance, it is evident that the above-mentioned problem is caused in that part.
Furthermore, this uneven density can be thought to be naturally eliminated by suppressing a conveyance deviation of the printing medium as much as possible. However, the deviation generated on the apparatus like this is hard to be eliminated completely, therefore, the displacement of several 10 μm or so tends to be generated while conveying the printing medium. On the other hand, as the distance between the plural ejection opening arrays is made to be shortened relatively, the uneven density become not conspicuous because a location displacement influence of the impacting is reduced. However, the distance between the ejection opening arrays is hard to be shortened from a consideration of arrangement of the ejection opening, a wiring layout of the printing element provided in the ejection opening, securement of a space portion in which the ink jet printing head and a cap protecting the ink jet printing head may contact each other, and the like.